Two former Parramatta Eels players are accused of harbouring semi-automatic weapons and possessing more than half-a-million dollars in cash after dramatic arrests in Sydney's Centennial Park yesterday.

Sydneysiders seeking respite at the Wet'n'Wild water park in the city's west about lunchtime instead found themselves sweltering in heavy traffic, the Transport Management Centre said.

The car park at the water park was full by 1pm (AEDT).

By early evening some holidaymakers were still experiencing lengthy traffic snarls.

Along the Pacific Highway between Woolgoolga and Emerald Beach, southbound motorists were queued for five kilometres before 6pm (AEDT).

Despite a feared spike in accidents at the state's beaches and backyard pools, paramedics said people had heeded warnings to look out for children near the water and to avoid drinking before taking a dip.

A NSW Ambulance Service spokesman told AAP there were no drownings or near-drownings.

"But we do suffer a spike in heat-related jobs on days like this," he said.